As the function of a pipe is to carry a volume of fluid from one point to another, the flow volume passing through it determines how well it is functioning. We will refer to fluid-carrying pipes as “vessels”, as in an exemplary application of measurement of flow in blood vessels. However, the teaching of how to measure flow volume in the present invention is applicable to measurement of flow of any liquid that contains particles that scatter ultrasound, e.g. milk, slurries, water containing bubbles, etc, as well as blood. Using ultrasound Doppler techniques to measure the flow of blood is well known. Red blood cells act as scatterers of ultrasound in the MHz frequency region, and when they are insonated by a beam of ultrasound their movement creates a Doppler shift in the scattered sound. The amount of shift in frequency, also known as the Doppler shift, is proportional to the number of wavelengths of ultrasound per second that the red blood cell moves. This proportionality is the cosine of the angle between the velocity of the scatterer and the direction of propagation of the ultrasound beam. As the peak velocity of blood in human blood vessels is about 1 meter/second, using ultrasound in the low MHz, where the wavelength is a fraction of a millimeter, leads to Doppler shifts in the low KHz, i.e. in the audible region, in which detected signals can be heard. By detecting the Doppler shifts, the velocity of the blood cells can be calculated. See, for example “Doppler Ultrasound” by Evans and McDicken, 2nd Ed, J. Wiley and Sons, New York 2000, for a thorough discussion of the use of Doppler ultrasound in measuring blood velocity.
Doppler velocity measurements are usually made with a combination of an image of the vessel with a graphic presentation of the Doppler shift vs time, known as “duplex Doppler”. The translation of the measured Doppler shift to the more useful velocity generally assumes the flow to be parallel to the axis of the vessel. Other techniques that have been proposed require multiple frequencies or complex mathematical manipulations of the signal. Conventional ultrasound methods often sample only a small portion of the flow through a vessel and extrapolate a flow from that small sample. This, however, frequently causes measurement to be inaccurate.
The present invention provides an apparatus and method to overcome these drawbacks in the existing Doppler measurement art. In the present invention, we teach a new configuration for direct application to the vessel to allow accurate measurement of flow carried by the vessel. Because rotational symmetric cylindrical transducers are used, this configuration will produce Doppler signals only from the flow directions down the tube, i.e. parallel to the axis; these flow components are the ones which are significant for determining the volume actually conveyed by the vessel. Moreover, unlike conventional ultrasound methods that sample only a small portion of the flow through a vessel and extrapolate a flow volume from that small sample, the present invention measures flow through most of the cross-section of the lumen, thus can provide an accurate measurement.